What is Cross-Site Scripting?

Cross-site scripting (XSS) is a type of web security flaw that lets an attacker mess with how you interact with a website. It’s like a digital chameleon, allowing attackers to slip malicious code into trusted websites and cause chaos for you. This article will help you understand XSS vulnerabilities, their impacts, and how to defend against them.

Defining Cross-Site Scripting

Cross-site scripting (XSS) is a security vulnerability found in web applications that lets attackers inject malicious code into the content that you view. These attacks happen when a web application uses input from you to generate output without properly validating or encoding it.

This means that if a website takes information directly from a search query or comment section and shows it on another page, it might accidentally run some malicious code. This injected code then runs in your browser, potentially letting the attacker steal information, change the look of the site, or even redirect you to another site. Understanding XSS is essential for anyone involved in web development or using web applications, as it’s a common yet dangerous threat.

How Does XSS Work?

The way this works is pretty clever, actually, but it can be broken down into a series of steps. So, to better understand it, here’s a breakdown:

• First off, there is an untrusted input: attackers use input fields, URLs, or any part of a web request to inject their malicious code, which is often JavaScript. The data is not validated properly, as this is the initial vulnerability that attackers use to launch XSS attacks.
• Now, comes the vulnerable application: the targeted web application doesn’t properly sanitize this injected data. Thus, when it generates an output, such as a web page, it blindly includes the malicious script. This is where the injected data gets mixed with the application’s legitimate content.
• Execution comes next: eventually, when you visit the compromised web page, your browser executes the injected script. The browser can’t tell the difference between safe and unsafe scripts, so it runs everything. Because the injected script runs within your browser in the context of the website, it can perform actions as if it were part of the trusted site.
• Compromise of user interactions is the last step: afterwards, once the script is running, the attacker can now steal your session cookies, redirect you to phishing sites, or even modify the HTML content you’re seeing, thus compromising your interactions with the web application.

Because XSS attacks exploit vulnerabilities, it’s essential to understand how they occur and what to do to defend against them.

Reflected XSS

Reflected XSS, also known as non-persistent XSS, is a type of attack where the malicious script bounces off a web server, plain and simple. Think of it like this: the injected script is included in your request, and the server unwittingly reflects it back to you in the response. So, let’s look at how this whole process works:

• Initially, the attack starts with a crafted URL: criminals create a URL that includes a malicious script as part of a query parameter. This URL is then distributed to potential victims through emails, social media, or other methods.
• Next, the user clicks the malicious link: once you click the malicious link, your web browser sends a request to the server, including the embedded script as part of the request.
• However, the server doesn’t sanitize the data: unfortunately, the server processes the request and includes the unsanitized script in the response it sends back to you. There is no escaping, encoding, or validating the data before sending the content back.
• Finally, the victim’s browser executes the script: unsuspecting and unaware, your browser receives the response and executes the script contained within it. This script can steal cookies, redirect you to malicious websites, or perform other harmful actions.

Reflected XSS attacks are sneaky because the malicious script is not stored anywhere; it’s immediately returned in the response.

Stored XSS

Stored XSS, also known as persistent XSS, involves injecting malicious code that gets permanently saved on a web server. This can happen through comment sections, forums, or any input field that stores data.

Once the malicious script is stored, it affects anyone who views the compromised content. Let’s break down how a stored XSS attack unfolds.

• First, the attacker injects a malicious script: an attacker submits malicious code into a website, and the code is stored in the website’s database. User profiles, forums, and review sections are common injection points.
• Then, the application stores the script: the web application saves the malicious script in its database without properly encoding or sanitizing it. This act of storage makes the threat “persistent”.
• Ultimately, users trigger the attack unknowingly: any user who visits the page where the malicious script is stored will have the script executed by their browser. Again, this can lead to cookie theft, redirects, or other malicious actions.

Stored XSS is more dangerous than reflected XSS because the malicious script doesn’t require a crafted URL. It’s automatically executed once a victim visits the infected page.

DOM-Based XSS

DOM-based XSS is a client-side attack, which means the vulnerability exists in the JavaScript code running on your browser, not on the server itself. This occurs when JavaScript improperly handles data from a source like a URL or cookie and writes it to the Document Object Model (DOM). Once that occurs, the script allows for execution of malicious code. Now, here is a little about how it works.

• Firstly, the user action is triggered by JavaScript: DOM-based XSS occurs when client-side JavaScript reads data from a controllable source, such as the URL, and writes it to a “sink,” a part of the DOM that supports JavaScript execution.
• Additionally, the server plays no part in the exploit: the server doesn’t validate or sanitize the data, as the exploitation happens entirely on the client side. This makes it harder to detect and prevent with traditional server-side security measures.
• The malicious script executes in the browser: when the browser renders the page, the injected script executes, leading to potential cookie theft, redirects, or other malicious actions.

This kind of XSS is tricky because it never sends malicious data to the server, making it challenging to detect with server-side security measures.

XSS Attack Consequences

Once a cybercriminal injects a cross-site scripting vulnerability is typically able to do the following:

• Masquerade or impersonate users: If an attacker gains your session cookie through XSS, they can impersonate you. Impersonation of a user can have damaging effects such as stealing sensitive data, or tarnishing a user’s reputation.
• Take any action that is able to be performed: an attacker can use XSS to perform actions on your behalf, such as changing settings or making purchases. In essence, if you are able to do it, so can they.
• Gain access to all data: what’s worse, all the data you can access can be stolen. In some instances, attackers can target data related to the website or other users too.
• Capture login credentials: some attackers may use XSS to set up fake login screens that trick you into giving up your login credentials. With that information, attackers can then take control of your account and perform actions on your behalf.
• Change web content: even worse, some malicious individuals can inject content into the website. Sometimes attackers can cause damage to an organization by creating distrust in customers if they happen to land on the altered page.
• Inject Trojan function into a web site: an attacker could inject a Trojan function into the website, meaning that the legitimate website that you thought you were on could be compromised.

These impacts make it critical to implement strong XSS prevention measures, as a single vulnerability can lead to widespread compromise.

How to Test for XSS Vulnerabilities

Web applications can implement certain techniques to test for XSS vulnerabilities, which are as follows:

• Submitting unusual input is a first step: insert unique strings into input fields and URL parameters to observe how the application processes and reflects this input.
• Checking HTTP Responses: monitor the HTTP responses to see if the submitted input is returned in the HTML without proper encoding.
• Validating Input Parameters: analyze different parameters, including POST data, headers, and cookies, to understand how each handles input and adjust your testing strategy accordingly.
• Exploiting the use of automated tools: use vulnerability scanners to find reflected and stored XSS issues.
• Using DOM Analysis: with DOM Invader, a Burp Suite extension, finding DOM-based XSS vulnerabilities is relatively easy. Use the traditional method of testing for DOM-based vulnerabilities by manually exploring the client-side code, which can be time-consuming.

You can prevent a costly data breach by checking for XSS vulnerabilities.

Securing Cookies

Cookies are small pieces of data stored on your computer by websites you visit, which are often used to remember login information or preferences. Given that, cookies can be a target for cross-site scripting (XSS) attacks. The following techniques enhance cookie security and reduce the risk of XSS-related cookie theft:

• First, use the HTTPOnly attribute: prevent client-side scripts from accessing cookies by setting the HTTPOnly attribute. This makes sure that only the server can access the cookie, which is one of the primary means of protecting information from being stolen.
• Then, use the Secure attribute: enforce the transmission of cookies only over HTTPS by setting the Secure attribute. This ensures that the cookie’s contents are encrypted during transmission.
• And use the SameSite attribute: limit cookie sharing between sites to prevent CSRF attacks, setting SameSite to Strict or Lax. This reduces the risk of unauthorized access by attackers on other sites.
• Implement cookie length and rotation: change cookies frequently, which could lower the amount of time an attacker has to take advantage of a stolen session. Shorter cookie lifetimes add an extra layer of safety to your cookies, which would improve your cybersecurity.
• Additionally, validate and sanitize data: always, always validate and sanitize any user-supplied data. This is because XSS is reliant on data being submitted by users. Therefore, it makes sense to eliminate the problem at the source.
• And use an encoding system: to make sure that data is being passed safely, a great option is to encode data. The most basic version involves implementing an HTML entity encoding.

Content Security Policy

Content Security Policy (CSP) is a browser mechanism that helps mitigate the impact of cross-site scripting (XSS) attacks, which offers a defense mechanism to manage scripts that are able to load in the browser.

By whitelisting origins, it restricts the sources from which the browser is allowed to load resources, enhancing your application’s security. So, to further understand Content Security Policy, let’s consider what needs to be done to properly implement it.

• First, implement response headers: properly configure CSP headers to instruct the browser on which sources to trust. The Content-Security-Policy HTTP header will determine how your content is able to be displayed.
• Then, define directives accurately: use CSP directives such as script-src, style-src, and img-src to define which sources are trusted for scripts, styles, and images, and ensure that they align with the application’s needs. By limiting the sources your content is pulled from, you can reduce the attack vector in a breach.
• However, avoid using ‘unsafe-inline’: it is recommended to avoid ‘unsafe-inline’ to help protect against the execution of malicious code that is injected into your site. However, if there is inline JavaScript needed to be used, use a cryptographically-secure nonce.
• Consider using ‘nonce’: you could also generate a unique cryptographically-secure nonce (‘number used once’) for each request. This helps you make sure that scripts in the HTML document are safe to run and that untrusted authors are not creating problems for you.
• Finally, routinely test policies: routinely check and change CSP policies as needed for an application. As applications evolve and technologies change, it may be necessary to edit your policies to match the current environment.

CSP is a powerful tool in mitigating XSS attacks; however, it requires careful configuration to avoid unintended consequences.

Summary

Cross-site scripting (XSS) is a web security vulnerability that allows attackers to inject malicious scripts into trusted websites that can compromise the interactions that you have with a site. Ranging from stealing cookies to redirecting your browser to a malicious website, the consequences of XSS can be severe, impacting both you and the web application.

By validating input, encoding data, and using methods such as HTTPOnly and Content Security Policy, developers are equipped with methods of protecting their online assets and providing safe experiences to all users. After all, knowing is half the battle, and now you are better equipped to wage war against cross-site scripting.